A standard overpressure control valve, as described in German patent document No. 2,915,280, is of the holding type, that is once it opens it stays open. The valve has an output chamber connected to the downstream line, an intake chamber connected to the upstream or control line, and a valve seat between these chambers. A valve body can be moved between a closed position on the seat and preventing fluid flow between the chambers and an open position permitting such flow.
Such a valve has a spring urging it into the open position. The pressure of the intake chamber is somehow effective on the valve body in a direction opposite that of the spring force, so that when this pressure exceeds a predetermined level the valve will close. If the forces were reversed the arrangement would be an underpressure valve that closes when the pressure on one side drops below a predetermined level.
Such a valve is not effective, once it closes, if the pressure upstream drops too much, in which case it will open. Thus when, for instance, installed in a gas main to protect downstream users from dangerous overpressures, it will open if the upstream overpressure valve itself closes to depressurize its upstream side. Opening under these circumstances can be dangerous.
Another such valve is described in French Pat. No. 1,060,914. It similarly uses the pressure in the line to hold it open. A disadvantage of this system is that when the pressure drops the valve opens at virtually the same pressure at which it closed. What is more, such a valve cannot be used to control pressure in the downstream line, only to cut if off from the upstream line, and cannot possible control pressure in a downstream line in accordance with that of an upstream line.
As mentioned above, such valves are installed in natural-gas mains to prevent the pressure therein, which can be as much as 15bar, from rising above a predetermined level. This is done to protect downstream users of the gas and makes transportation through the pipeline more efficient. Normally such a gas main is provided with main valves that subdivide it into sections and with pressurizers to keep the pressure up. In addition so-called safety valves close the line when the pressure downstream drops below a predetermined lower level. The control valves of the instant invention prevent the lines from being overpressurized, as for instance in the vent of pressurizer failure.
The standard control valve is occasionally subjected to considerable pressures, as when a compressor upstream does not shut down at the upper pressure limit. Thus these valves are periodically acted on by forces far above their nominal ratings. When they have rotary actuators they can become wedged on or off, and will be subject to great stress and wear in the event of a serious upstream overpressure.